Atmospheric Pressure Glow (APG) discharge is used in practice, among others, for non-destructive material surface modification. Glow discharge plasmas are relatively low power density plasmas, typically generated under atmospheric pressure conditions or partial vacuum environments.
Most commonly, the plasma is generated in a plasma chamber or plasma discharge space between two oppositely arranged parallel plate electrodes. However, the plasma may also be generated using other electrode configurations such as, for example, adjacently arranged electrodes. The plasma is generated in a gas or a gas mixture by energizing the electrodes from AC power supply means.
It has been observed that a stable and uniform plasma can be generated in, for example, a pure Helium or a pure Nitrogen gas. However, as soon as impurities or other gasses or chemical compositions at ppm level are present in the gas, the stability of the plasma will decrease significantly. Typical examples of stability destroying components are O2, NO, CO2, etc.
Instabilities in the plasma will either develop in a high current density plasma or will extinguish the plasma locally. With a large density of species and a high frequency of collisions in the plasma, an APG shows a fast positive feedback. That is, a random local increase of the ionization of the plasma will exponentially increase. Accordingly, an instability will develop either in a high current density plasma or will extinguish locally the plasma. This phenomenon of exponential increase of the plasma current is known as glow to arc transition. As a result, current arcing occurs and the glow discharge plasma can not be maintained. Instead, a combination of filamentary and glow discharge is generated.
Filamentary discharge between parallel plate electrodes in air under atmospheric pressure has been used to generate ozone in large quantities. However, filamentary discharge is of limited use for surface treatment of materials, since the plasma filaments tend to puncture or treat the surface unevenly and are associated with relatively high plasma current densities.
Although instabilities may occur at any time during generation of a plasma, it has been observed that the circumstances at the end of a plasma pulse (e.g. generated using an AC voltage) are beneficial to the occurrence and development of instabilities. At the end of the plasma pulse, the plasma current is relatively low while the voltage applied by the AC power supply increases due to the fact that the AC power supply tends to recover from the main plasma pulse. The life cycle of the plasma is approaching cut off in this stage of the plasma and the plasma current will decrease sharply, after which small instabilities and plasma variations may occur. Under the influence of the increasing voltage resulting from the recovering AC power supply, these instabilities and plasma variations may easily develop in major plasma instabilities, such as streamer formation and glow to arc transition. These circumstances, relatively low and decreasing plasma current together with increasing voltage, make it at the same time difficult to suppress or eliminate the instabilities.